Memory devices are typically internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Unlike most semiconductor memory, flash memory devices can be sold with a limited quantity of defective memory blocks. Flash memory devices typically have a scheme to communicate the locations of the defective blocks to a controller or processor.
One such scheme employs markers at specific locations in the defective memory block. The markers are generated during the manufacturing and testing process before being sold to the user. When the user powers-up the memory device, the memory controller would go through these specific locations in memory and track the defective blocks. Since there can be over 2000 memory blocks in a typical memory device, this is considerable overhead for the controller to handle.
In another error detection scheme, the memory controller reads the memory cells in a memory block and if the data is anything but FFH it would consider that block as a bad block. The controller then moves on to the next block. When all of the block's memory locations are read as FFH, the controller considers that block as good and starts to extract the information that it expected to see in block 0.
However, if the memory defect is such that the controller is incapable of writing to any location in that block, the FFH data could not be altered from the initial erased condition. In this case, the controller would assume that the block is good since it read an FFH. Upon trying to write desired data into this block, the system would not allow it due to the write defect.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved method for handling defective memory locations with minimal controller overhead.